1. Field of the Invention
This invention relates to a drive circuit to be suitable used for driving an electroluminescence (EL) display device.
2. Prior Art
A circuit diagram of a conventional drive circuit for an EL display device is illustrated in FIG. 5 of the accompanying drawings. Such a drive circuit typically comprises a power supply section 1, an oscillator section 2 and a voltage booster section 3. Stabilization of the DC power source E in the power supply section 1 is achieved by a condenser C.sub.1. The oscillator section 2 comprises a transistor Q.sub.1, condensers C.sub.2 and C.sub.3, resistors R.sub.1 and R.sub.2. The voltage booster section 3 is constituted by a transformer T. In a drive circuit having a configuration as described above, the oscillation output of the oscillator section 2 is boosted to a predetermined level (approximately 100 V) by the transformer T and fed as drive voltage to an EL display device 4. The oscillator section 2 is influenced by resonance frequency f.sub.0 which is defined in terms of capacitance C of the EL display device 4 and reactance L of the transformer T or ##EQU1## and forcedly oscillates with the resonance frequency f.sub.0. Therefore, the values of resistance of the resistors and those of capacitance of the condensers involved are so determined that the oscillation frequency of the oscillator section 2 agrees with the resonance frequency f.sub.0. In other words, the EL display device 4 is driven by this drive circuit by using only one frequency.
Generally, blue-green light emitting fluorescent powdery materials used for dispersion EL display devices have properties as described below.
(1) As the drive frequency becomes high, the color of the emitted light shifts from green to blue.
(2) As the involved material is degraded with time through use, the color of the emitted light shifts from blue to green if the drive frequency is kept unchanged.
Now, if an EL display device 4 is driven by a drive circuit as illustrated in FIG. 5, the display device will be degraded with time and the capacitance C of the display device 4 will be varied in response to the degradation to consequently shift the drive frequency to a higher level. Accordingly, the color of the emitted light of the EL display device 4 will shift with time in a manner as described in (1) and (2) above or from blue to green. FIG. 6 is a graphic illustration of the shift of the color of the emitted light with time expressed by using chromaticity coordinates (x, y) defined by the CIE.
There has been known a drive circuit comprising a transformer having a vertically adjustable core of a coil for regulating the color of the emitted light of an EL display device. Such a drive circuit can be used for an EL display device by adjusting the vertical position of the core of the coil to match the correct color of the display device.